1. Field of the Invention
The present invention relates to a new process for preparing aliphatic polyisocyanates having iminooxadiazinedione groups.
2. Description of Related Art
Polyisocyanates containing iminooxadiazinedione groups are raw materials which are of great interest for the production of polyurethane plastics and polyurethane coatings. Their preparation, described for example in EP-A 0 798 299, is accomplished using catalysts of saltlike composition whose anion contains fluorides, F−, or di- or polyfluorides, [F—H—F]− or [(F—H)n—F]− n>1, respectively.
In the latter cases it is necessary, for the purpose of catalyst preparation, to operate with hydrogen fluoride (HF), which because of the unpleasant physiological properties of the compound signifies a considerable safety expense and, moreover, is not universally possible technically. Catalysts prepared in an HF-free way, i.e. catalysts containing only fluoride anions, tetraalkylammonium fluorides or tetraalkylphosphonium fluorides, for example, have a storage stability which is poor, particularly in concentrated solution. They tend to discolor, which can have deleterious consequences for the quality of the polyisocyanates prepared using them. Although highly dilute solutions are more stable, they are poorly suited to the intended use in isocyanate trimerization with preferential formation of iminooxadiazinedione groups, since dilution is detrimental to the iminooxadiazinedione selectivity.
The use of compounds containing carboxylate groups in NCO oligomerization is known, although in that case typically the formation of isocyanurate is observed (J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200). The use of fluorinated carboxylate anions has not been described in this context.
It is an object of the present invention to provide alternative catalyst systems, i.e. fluoride or di- and polyfluoride-free catalyst systems, for the preparation of polyisocyanates containing iminooxadiazinedione groups.
This object may be achieved according to the invention by the use of the saltlike catalysts described hereinafter.